First-principles calculation of alloy scattering in Ge(x)Si(1-x)

First-principles electronic structure methods are used to find the rates of intravalley and intervalley n-type carrier scattering due to alloy disorder in Si(1-x)Ge(x) alloys. The required alloy scattering matrix elements are calculated from the energy splitting of nearly degenerate Bloch states which arises when one average host atom is replaced by a Ge or Si atom in supercells containing up to 128 atoms. Scattering parameters for all relevant Delta and L intravalley and intervalley alloy scattering are calculated. Atomic relaxation is found to have a substantial effect on the scattering parameters. f-type intervalley scattering between Delta valleys is found to be comparable to other scattering channels. The n-type carrier mobility, calculated from the scattering rate using the Boltzmann transport equation in the relaxation time approximation, is in excellent agreement with experiments on bulk, unstrained alloys.     

应变Si/(001)Si1-xGex电子迁移率

依据离化杂质散射、声学声子散射和谷间散射的散射模型,在考虑电子谷间占有率的基础上,通过求解玻尔兹曼方程计算了不同锗组分下,不同杂质浓度时应变Si/(001)Si_1-xGe_x的电子迁移率.结果表明:当锗组分达到0.2时,电子几乎全部占据Δ₂能谷;低掺杂时,锗组分为0.4的应变Si电子迁移率与体硅相比增加约64%;对于张应变Si NMOS器件,从电子迁移率角度来考虑不适合做垂直沟道.选择相应的参数,该方 关键词： 电子谷间占有率 散射模型 锗组分 电子迁移率     

Intervalley-scattering-induced electron-phonon energy relaxation in many-valley semiconductors at low temperatures

We report on the effect of elastic intervalley scattering on the energy transport between electrons and phonons in many-valley semiconductors. We derive a general expression for the electron-phonon energy flow rate at the limit where elastic intervalley scattering dominates over diffusion. Electron heating experiments on doped n-type Si samples with electron concentrations (3.5-16.0) x 10(25) m(-3) are performed at sub-Kelvin temperatures. We find a good agreement between the theory and the experiment.     

Optical and intervalley scattering in quantized inversion layers in semiconductors

The momentum relaxation time for scattering of electrons in quantized levels of an inversion layer on a semiconductor surface is calculated for interactions via optical and intervalley phonons. A selection rule is found which prohibits transitions between subbands belonging to the same valley or set of valleys, at least in the zero order to which these scattering processes may occur. Relaxation times for the zero-order interaction and the first-order interaction are obtained for intervalley phonons. The results are applied to the case of a (100)-silicon surface, with electrons in the three lowest subbands (with energy levels E₀, E₁, E'₀) of the two sets of valleys. Agreement with the experimental data of Fang and Fowler is good when the combined effects of intervalley and acoustic scattering are considered.     

Photoemission of graded-doping GaN photocathode

We study the photoemission process of graded-doping GaN photocathode and find that the built-in electric fields can increase the escape probability and the effective diffusion length of photo-generated electrons,which results in the enhancement of quantum efficiency.The intervalley scattering mechanism and the lattice scattering mechanism in high electric fields are also investigated.To prevent negative differential mobility from appearing,the surface doping concentration needs to be optimized,and it is calculated to be 3.19×10 17 cm 3.The graded-doping GaN photocathode with higher performance can be realized by further optimizing the doping profile.     

Galvanomagnetic effects in n-type gallium antimonide

This paper reports the theoretical and experimental study performed on Hall mobility and free carrier Faraday rotation in a degenerate n-GaSb sample in the temperature range 77–300K. Following relaxation time approximation the mobility is estimated separately for the Γ- and L-valley taking into account the scattering of electrons due to ionised impurities, space charge, polar optical phonons, deformation potential, intervalley acoustic and optical phonons. The effective mobility is calculated considering a two valley model, degeneracy and non-parabolicity of the Γ-valley, and compared with these experimental results. Microwave Faraday rotation data at 77 and 300K is analysed generalising the d.c. magneto-conductivity tensor components as derived by Bordure and Savelli to the high frequency, and is used to confirm the scattering mechanisms and band parameters used in the analysis of d.c. galvanomagnetic results.     

Inversion layer mobility with intersubband scattering

The acoustic-phonon limited electron mobilities are calculated for the Si(100) inversion layer, taking, for the first time, the intersubband, intravalley transitions fully into account, yet leaving the intervalley scattering for future studies. The results represent a major improvement over the previous single subband theories. There are indications that the discrepancies still remaining between our theory and experiments will disappear if the intervalley scattering is taken into account properly considering the multi-subband structure.     

纤锌矿相GaN空穴输运特性的Monte Carlo模拟研究

用全带多粒子Monte Carlo方法模拟纤锌矿相(Wurtzite)GaN空穴输运特性的结果. 用经验赝势法计算得到能带结构数据. 模拟包含了声学声子散射，光学声子散射，极性光学声子散射，压电散射，电离杂质散射及带间散射等散射机理. 计算得到了空穴沿3个主要对称方向上的空穴平均漂移速度和平均能量与电场强度的关系曲线，室温下漂移速度呈现饱和特性. 在所研究的电场范围内，最大平均漂移速度约为6×10⁶cm s^-1，最大空穴平均能量约为0.12eV, 这些值均比电子的相应参数低很多. 还给出了空穴的扩散迁移率与杂质浓度关系的模拟结果. 关键词： 蒙特卡罗 氮化镓 输运特性 能带结构     

First order intervalley scattering in silicon inversion layers

The mobility limited by first order intervalley scattering in a quantized silicon inversion layer is calculated by an iterative solution of the Boltzmann equation incorporating both scattering-out and scattering-in terms. The mobility values are found to be much smaller than those based on a relaxation-time model considering scattering-out only.     

Calculation of hot electron distribution functions and of the threshold field in the gunn effect

The Boltzmann equation is formulated for the (000) electrons in GaAs in the highenergy approximation taking into consideration acoustic, polar and nonequivalent intervalley scattering to the 〈100〉 satellite valleys and the nonparabolicity of the central valley. The simplified form of the equation permits a qualitative discussion of the shape of the electron density distribution without explicitly computing the distribution function and illustrates the effect of the various scattering mechanisms as a function of the electric field. Examples of electron density distribution curves are calculated for a few typical field values, and the result is used to derive the fielddependent population of the satellite valleys and the average drift-velocity in the vicinity of the Gunn threshold. For an intervalley deformation potential constant of 5×10⁸ eV/cm the threshold is found to be 2.7 kV/cm in the parabolic case increasing to 3.5 kV/cm when nonparabolicity is included. Comparison with experimental data is made.     

Hot electron photoluminescence in GaAs crystals

The spectrum and the linear polarization of photoluminescence of hot electrons in GaAs crystals were investigated. Oscillations in the hot photoluminescence (HPL) spectrum due to the subsequent emission of LO-phonons were observed. The study of HPL depolarization in an external magnetic field yielded the scattering time due to the emission of a LO-phonon by a hot electron in the Γ-valley (τ_?0 = 1 × 10^?13 sec) as well as the Γ?L intervalley scattering time. The radiative recombination of hot electrons created in the central Γ-valley via the subsidiary L-valley was observed. The distribution function of hot electrons in a wide energy range was evaluated from the spectra.     

Hot electrons in Si inversion layer

The energy losses per electron to phonons (both acoustic and optical) are calculated for Si(100) inversion layer taking the subband structure of electron energy spectrum fully into account. The electron mobility variations with applied field are studied by energy balance method. We examine two extreme cases: (1) the intervalley transitions connect any pair of subbands in respective valleys with the same transition matrix; (2) the intervalley transitions connect only the lowest subbands in each valley. It turns out that we can choose the magnitude of intervalley transition amplitude such that experimental data lie in between the theoretical results in two cases mentioned above at both lattice temperatures T = 77 and 300 K.     

单轴[110]应力硅电子迁移率

基于k·p微扰法研究单轴[110]应力作用下硅的导带结构,获得单轴[110]应力硅的导带底能量及电子有效质量.在此基础上,考虑电子谷间、谷内及电离杂质散射,采用弛豫时间近似计算单轴[110]应力硅沿不同晶向的电子迁移率.结果表明：单轴[110]应力作用下硅的电子迁移率具有明显的各向异性.在[001]、[110]及[110]输运晶向中,张应力作用下电子沿[110]晶向输运时迁移率有较大的增强,由未受应力时的1 450 cm²·Vs^-1提高到2 GPa应力作用下的2 500 cm²·Vs^-1.迁移率增强的主要原因是电子有效质量的减小,而应力作用下硅导带能谷分裂导致的谷间散射几率的减小对电子迁移率的影响并不显著.     

Electronic transport in quasicrystals

Mobility of electrons in quasicrystals is considered in the framework of the fractional Fermi surface (FS) model, i.e., a multiconnected FS with many electron-hole pockets. The Mott law for the variable range hopping conductivity is obtained when intervalley scattering processes with small momentum transfer are taken into account. The transition to the power-law temperature dependence is discussed.     

Determination of X6 valley hot electron dynamics and the intervalley X6 → Γ6 scattering time in GaAs

The time-resolved infrared absorption in a GaAs crystal was measured by picosecond visible-pump and infrared-probe absorption spectroscopy. The dynamics of hot electrons in the X₆ valley subsequent to the excitation was obtained. From the time evolution of the population of the X₆ electrons, the intervalley X₆ → Γ₆ scattering time was determined to be ≈ 500 fs by a rate equation analysis.     

Temperature and electric field dependences of the mobility of electrons in vertical transport in GaAs/Ga<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>As barrier structures containing quantum wells

The mobility of electrons in vertical transport in GaAs/Ga_1−y Al _y As barrier structures was investigated using geometric magnetoresistance measurements in the dark. The samples studied had Ga_1−y Al _y As (0 ≤ y ≤ 0:26) linearly graded barriers between the n⁺-GaAs contacts and the Ga_0:74Al_0:26As central barrier, which contain N _w (=0, 2, 4, 7 and 10) n-doped GaAs quantum wells. The mobility was determined as functions of (i) temperature (80–290 K) at low applied voltage (0.01–0.1 V) and (ii) applied voltage (0.005–1.6 V) at selected temperatures in the range 3.5–290 K. The experimental results for the temperature dependence of low-field mobility suggest that space-charge scattering is dominant in the samples with N _w =0 and 2, whereas ionized impurity scattering is dominant in the samples with N _w =4, 7 and 10. The effect of polar optical phonon scattering on the mobility becomes significant in all barrier structures at temperatures above about 200 K. The difference between the measured mobility and the calculated total mobility in the samples with N _w =4, 7 and 10, observed above 200 K, is attributed to the reflection of electrons from well-barrier interfaces in the quantum wells and interface roughness scattering. The rapid decrease of mobility with applied voltage at high voltages is explained by intervalley scattering of hot electrons.      

Ab initio method for calculating electron-phonon scattering times in semiconductors: application to GaAs and GaP

We propose a fully ab initio approach to calculate electron-phonon scattering times for excited electrons interacting with short-wavelength (intervalley) phonons in semiconductors. Our approach is based on density functional perturbation theory and on the direct integration of electronic scattering probabilities over all possible final states with no ad hoc assumptions. We apply it to the deexcitation of hot electrons in GaAs, and calculate the lifetime of the direct exciton in GaP, both in excellent agreement with experiments. Matrix elements of the electron-phonon coupling, and their dependence on the wave vector of the final state and on the phonon modes, are shown to be crucial ingredients of the evaluation of electron-phonon scattering times.     

Low temperature electron mobility in Ga0.5In0.5P/GaAs quantum well structures

We analyse the low temperature subband electron mobility in a Ga_0.5In_0.5P/GaAs quantum well structure where the side barriers are delta-doped with layers of Si. The electrons are transferred from both the sides into the well forming two dimensional electron gas (2DEG). We consider the interface roughness scattering in addition to ionised impurity scattering. The effect of screening of the scattering potentials by 2DEG on the electron mobility is analysed by changing well width. Although the ionized impurity scattering is a dominant mechanism, for small well width the interface roughness scattering happens to be appreciable. Our analysis can be utilized for low temperature device applications.      

Diamagnetism in quasicrystals

A reliable explanation of diamagnetism in quasicrystals is given. We show that the weak diamagnetism in perfect icosahedral quasicrystals is due to an atomic-like diamagnetic contribution of tightly bound conduction electrons in electron pockets of a multiconnected Fermi surface. The Landau-Peierls diamagnetic term is small due to large effective masses. At temperatures above the Debye temperature the intervalley electron-phonon scattering makes the electrons ‘free’, and the temperature dependence of the Pauli paramagnetism related to a pseudogap in the density of states at the Fermi level becomes important.     

Low temperature conductivity of doped semiconductors: Mass anisotropy and intervalley effects

The theory of the low temperature correction to the conductivity of weakly disordered three-dimensional systems due to electron-electron interaction effects is extended to the case of many valley systems (e.g., n-doped Si or Ge). A strong dependence due to both mass anisotropy and intervalley scattering is found, which is capable of explaining the large discrepancy between the experiment and existing theoretical estimates.